m/\s*(\S+)\s*(\S+)\s*(\S+)\s*(\S+)\s*/

##</code><code>##

$s1="ABC D E";
$s2=" ABCD E  ";  # (leading and trailing spaces)
print join( ":", split /\s+/, $s1 ), $/;
print join( ":", split /\s+/, $s2 ), $/;
print $/;
print join( ":", ($s1=~/\s*(\S+)\s*(\S+)\s*(\S+)\s*/)), $/;
print join( ":", ($s2=~/\s*(\S+)\s*(\S+)\s*(\S+)\s*/)), $/;

__OUTPUT__
ABC:D:E
:ABCD:E

ABC:D:E
ABC:D:E